OpenPOWER ISA > General Discussion

Learning POWER9 assembly

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MauryG5:
Guys excuse some curiosity, I read some time ago that the PowerPc instructions have been incorporated into the most recent Power ISAs, so theoretically, the old instructions that IBM and Motorola created specifically for PowerPC, are now also usable on the new Power, right? I ask because I remember that when PowerPc was born from Power, it eliminated some instructions from Power, which they said at the time were useless for the home desktop environment and instead inserted instructions made specifically for Power PC in its place ... Another curiosity that I I ask is related to floating point computation units such as the VSX and the like, deriving from the legendary Altivec. Because some people say, for example I have read some of these comments on Phoronix, that the floating point compute units of the Power processors are poor compared especially to the X86 equivalents! I wonder is such a thing ever possible when it was PowerPC's Altivec at the time, he taught Intel how to make floating point computing units, much less performing up to the SS2 version ... I don't explain this what, indeed, should be one of Power's greatest strengths, especially with the evolution that certainly all these units have had over the years ...

ClassicHasClass:
Almost all, but not every single instruction, of the PowerPC instruction set is in Power ISA. Obviously 601-specific instructions didn't make it past the 603, but also a couple oddballs like mcrxr aren't in Power ISA (instead there is, more recently, mcrxrx), and there are differences in cache line which affects some instructions like dcbz. These were also issues on the G5, which is more POWER4 than it is PowerPC. In general, however, the vast majority of user-level code will translate to the extent it is 64-bit aware.

MauryG5:
I understand, so we have a good part of those instructions available, certainly those 601s would not even make sense to date, I think they are too dated by now and that's right in the end. The article I read a few years ago was correct so fine. What about floating point vector and compute units instead, has Intel really outdone us in this area or are we still the best with Alti Vec's direct successors?

ClassicHasClass:
Vectors are still "limited" to 128 bit, though I would also argue that the efficiency improvements with 256 and 512 bit vectors are more questionable. A good scalar system is what Power ISA needed, and VSX is a big improvement (no more spilling to memory to exchange between FP registers and integer registers, for example). I use it heavily in the Firefox JIT.

MauryG5:
Understood, so you think that basically it is not really an advantage that of X86 that has vectors at 256 or 512 from what I understand ... Do you think we will be able in the end to have Firefox without those limits that up to now this Browser has on Power also compared to Chromium? I see that unfortunately I still can't go to all the sites like I do on Chromium ... I think it would be nice, given the work you do with so much passion on Firefox, to be able to finally package a version specially dedicated to us of Power, maybe diversifying some colors of the logo, for example instead of orange, use the fire red which in my opinion would represent a Power version well ...

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